Low-energy fuse or shock tubes are well known in the art. See, e.g., U.S. Pat. Nos. 3,590,739 and 4,328,753. Multi-layer shock tubes also are known from U.S. Pat. No. 4,328,753. In that patent the innermost layer of the tube was required to have adhesive affinity for a reactive substance which coats the inner surface of the innermost layer. The outermost layer had resistance to external damage. The inner layer was composed of SURLYN 1855 (registered trademark of DuPont), and the outer layer was a polyamide.
The tubes of the type described in U.S. Pat. No. 4,328,753 have been manufactured commercially for several years and contain either a single layer or an inner layer of SURLYN having a thickness generally of from about 0.8 millimeters or greater. U.S. Pat. No. 4,607,573 specifically discloses a multi-layered tube in which the inner tube is elongated or stretched before forming the outer tubular layer. The elongating or stretching is done to reduce the reactive material core load per unit length and the inner tube wall thickness. The inner tube is described as having an average inner diameter in the range of 0.017 to 0.070 inches and an outer diameter in the range of 0.034 to 0.180 inches. In other words, the wall thickness may range from 0.436 millimeter to 2.82 millimeters or more. Since SURLYN is a relatively expensive component of the tube, it is desirable and an object of the present invention to reduce the thickness of the innermost layer to less than about 0.3 millimeter in diameter. The outer layer(s) then can be formed with less expensive materials in thicker layers as desired for abrasion, cut and other resistance purposes.
Forming the multi-layer tube by co-extrusion enables the use of a much thinner inner layer than can be obtained by previous methods.